1. Field of the Invention
This invention is concerned with the preparation of high purity alkali metal halide single crystals.
2. Description of the Prior Art
Metal halide crystals are being used extensively as windows for high power lasers and other optical applications. Their use in laser applications is limited by rigid constraints on anion purity. The vibrational modes of anions are infrared active and often involve high absorption cross sections so that much less than 1 ppm is needed to achieve absorption coefficients below 0.001 cm.sup.-1 in the crystal (C. J. Duthler, J. Appl. Phys. 45 2668, 1974).
Among other anions found in alkali metal halides are OH.sup.- ions. These anions pose special problems because H.sub.2 O, their source, is ubiquitous. The oxygen-hydrogen vibration is active at 2 to 4 .mu.m and the oxygen-cation vibration is active at 9 to 10 .mu.m. The substitutional OH.sup.- dipole couples with the dipole consisting of a bivalent metal ion impurity and a metal ion vacancy ("Influence of OH.sup.- Ions on Infrared Absorption and Ionic Conductivity in Lithium Fluoride Crystals," by T. G. Stoebe, J. Phys. Chem. Solids 28, 1375, 1967).
Conventional prior art crystal growth processes have not been effective in reducing the level of OH.sup.- concentrations in the finished crystals.
The effect of OH.sup.- ions on the absorption characteristics of alkali metal halide single crystals has been long recognized and considerable resources have been devoted to solving this problem. The results of one study having as its objective the elimination of OH.sup.- ions from alkali metal halides to be used as starting materials in subsequent single crystal growth processes are reported in British Pat. No. 1,137,582, issued to Miroslav Lehl in December of 1968. According to the U.S. Pat. No. 1,137,582 disclosure, anions of a starting alkali metal compound other than halide is replaced by a halogen. Gaseous halides of carbon or sulfur, or a thionyl or carbonyl gaseous halide in an inert gas mixture are fed into a melt of the starting material at a temperature not to exceed 1250.degree. for a period ranging from 5 to 10 minutes. The gaseous mixture is removed, the ampoule containing the purified halide is sealed and then allowed to cool down. While the process is alleged to yield alkali metal halides which show no measurable infrared or ultraviolet absorption, it does not teach how to ultimately produce a single crystal halide which exhibits physical and optical characteristics suitable for laser applications in the 2-6 .mu.m and 10.6 .mu.m regions where OH.sup.- ion concentrations below a mole fraction of 10.sup.-6 are required.
The U.S. Pat. No. 1,137,582 process as it relates to repurification is defective in producing ultra-pure starting material to be used ultimately for crystal growth in that it contains steps which by their very nature tend to rehydrolyze the purified materials. Specifically, H.sub.2 O is released from the walls of the ampoule during the ampoule sealing step when there is no halogen purge present to getter the moisture. Alkali halides purified by this method are believed to contain slightly less than a mole fraction of 10.sup.-5 OH.sup.-, an amount greater by a factor of 10 than that required of the finished alkali metal halide crystal used in laser applications.
A similar material purification process is also disclosed in British Pat. No. 1,123,991, published Aug. 14, 1968. The U.S. Pat. No. 1,123,991 disclosure teaches a method of preparing alkali metal halides from alkali sulfates, sulfides, nitrides, hydrides, and carbonates by halogenating said compounds mixed with either powdered sulfur or carbon and heated beyond their melting points. In each of the examples disclosed, gaseous chlorine or bromine was fed into the melt of starting materials to achieve an anion exchange reaction. The resulting product contained unreacted traces of carbon or sulfur on the surface at the end of the process. This material preparation process is not nearly as effective as the purification process disclosed by Applicant in U.S. application Ser. No. 587,746, nor does the disclosure contain teachings which can be used to avoid the recontamination of the purified materials which occurs during subsequent handling in preparation for use in conventional crystal growth processes.
The largest supplier of ultra pure alkali metal halide single crystals is Harshaw Chemical Company of Ohio. Harshaw is believed to use a vacuum crystal growth process. A comparison of air-grown and vacuum-grown metal halide Harshaw crystals is discussed by L. S. Combes et al in "Mechanical and Thermal Properties of Certain Optical Crystalline Materials," J. Opt. Soc. 41 (No. 4), 215 (1951). An evaluation of Harshaw crystals is given by F. Horrigan, et al in "Windows for High Power Lasers," Microwaves, pp. 68-76 of Laser Technology Section (January 1969). Optical absorption at 10.6 .mu.m is reported to be greater than .1%/cm with an ultimate strength of 640 psi for KCl crystals.